A dysrhythmia is an abnormal heart beat pattern. One example of a dysrhythmia is a bradycardia wherein the heart beats at an abnormally slow rate or where significant pauses occur between consecutive beats. Other examples of dysrhythmias include tachyarrhythmias where the heart beats at an abnormally fast rate, e.g., atrial tachycardia where one or more atria of the heart beat abnormally fast. It is well known to treat such dysrhythmias with a pacemaker, an implantable cardiac defibrillator (ICD), or the like which delivers electrical stimulation pulses through one or more electrodes proximate to the distal end of one or more leads implanted within a patient's heart, i.e., endocardial leads. Many types of endocardial leads are known but they generally fit into two broad categories, passive fixation leads that use tines or the like to affix to the traebaculae in the patient's heart and active fixation leads that typically have a screw-in helix that screws into the myocardium. In either case, the mechanical trauma of implanting the endocardial lead will generally result in some degree of inflammation due to, among other things, foreign body reaction that can adversely affect the primary purpose of the implantation, that being to cause the heart tissue to contract by applying electrical stimulating pulses to the heart tissue under the control of the pacemaker. The ability of a pacemaker/ICD to stimulate the heart tissue depends upon overcoming a cardiac pacing threshold. A threshold value is related to the minimum amount of energy contained in a stimulation pulse of known amplitude and duration that is capable of stimulating the heart tissue. Typically, the threshold energy value following implantation, i.e., the acute stimulation threshold, is generally higher and decreases during the first few weeks after implant to a more stable chronic stimulation threshold value. It is well known that an endocardial lead can be made to elute a steroid, e.g., dexamethansone sodium phosphate or glucocorticosteroid, to reduce the amount of inflammation resulting from the implant and thus improve the capability of the pacemaker to stimulate the cardiac tissue with a decreased amount of energy, a limited resource in an implanted device. Accordingly, the battery life and the time between implants of a pacemaker/ICD will normally be extended.
Typically, the steroid is eluted from an endocardial lead electrode that contains a monolithic controlled release device (MCRD), in the form of a plug that is made of a mixture of a steroid, e.g., dexamethansone sodium phosphate, or equivalent, and a medical adhesive as a carrier for the steroid. The currently known and used medical adhesive/steroid combinations have limitations that add to manufacturing costs. Typically, the pot life, i.e., the time before the viscosity of the combination makes it difficult to handle (e.g., dispense, inject or spread), is relatively short, e.g., only about 10 minutes. Thus, the pot life limits the capability to mass-produce leads by dispensing or injecting the mixture into a completed or partially completed lead. In contrast, the curing time is typically relatively long, e.g., up to 24 hours. Accordingly, to accommodate for these characteristics, monolithic controlled release device (MCRD) plugs are typically manufactured outside the lead and inserted into the lead as an additional manufacturing step. To manufacture these plugs, a mixture is formed and rapidly (within the pot life limitation) spread over and into multiple cavities in a mold. Typically, the mixture is then cured in an oven at an elevated temperature, e.g., 50° C., for 2 hours and then air cured for an additional 8 hours. After curing is completed, e.g., 10 hours later, the cured plugs are extracted from the mold and subsequently inserted into the partially manufactured lead. This process typically results in a substantial waste of the steroid mixture (the portion that is not actually inserted into the mold cavities) as well as increasing manufacturing delays (due to the curing time), production steps, and the costs associated with each of these deficiencies. Therefore, it is very desirable to have a composition and process for forming a monolithic controlled release device (MCRD) that reduces these material and production costs. Accordingly, the present invention is directed to remedying the deficiencies of the prior art.